Search results

Rotating stall is an unsteady flow phenomenon that causes instabilities and low efficiency in pumps. The purpose of this paper is to investigate the rotating stall characteristics and unsteady behavior of stall cells in a centrifugal pump impeller at low flow rates.

A developed large eddy simulation with dynamic mixed nonlinear model is performed to evaluate the unsteady flow in a centrifugal pump impeller. The rotating stall flow field through the centrifugal pump impeller is analyzed under three typical flow rates. Frequency spectrum analysis are carried out on the series of pressure fluctuation to get the rotating stall characteristics. The size and intensity of stall cells are also analyzed using time-averaged vorticity and static pressure.

The rotating stall cell first occurs in the suction side of the blade and exhibits an obvious life cycle including decay mergence, shedding, growing and development with a low frequency. With the decrease of flow rate, the amplitude of pressure fluctuations in the impeller tends to be larger, the propagated speed of stall cells and rotating stall frequency tends to be smaller, but the number of cells remains unchanged. The size of stall cells increases as the flow rate decreases, but intensity changes is very little.

The rotating stall characteristics in a centrifugal pump impeller under low flow rates are presented first using a developed large eddy simulation approach.

Rotating flows are very important because they are found in industrial and domestic applications. For a good performance, it is important to dimension correctly the energy efficiency and the lifespan of the apparatuses while studying, for example, the influence of their physical and geometrical characteristics on the various hydrodynamic constraints, thermal and mechanics which they will support. The purpose of this paper is to describe experiments and a numerical study of the inter‐disc space effects on the mean and the turbulent characteristics of a Von Karman isotherm steady flow between counter‐rotating disks.

Experimental results are obtained by the laser Doppler anemometer technique performed at IRPHE (Institute of Research on the Phenomena out Equilibrium) in Marseille, France. The numerical predictions are based on one‐point statistical modeling using a low Reynolds number second‐order full stress transport closure (RSM model).

It was found that the level of radial velocity increases with the aspect ratio near to the axis of rotation but this phenomenon is reversed far from this zone; the level of tangential velocity, of turbulence kinetic energy and of the torsion are definitely higher for the largest aspect ratio. The best contribution of this work is, at the same time, the new experimental and numerical database giving the effect of the aspect ratio of the cavity on the intensity of turbulence for Von Karman flow between two counter rotating disks.

The limitation of this work is that it concerns rotating flows with very high speeds because the phenomena of instability appear and the application of this model for cavities of forms is not obvious.

This work is of technological interest; it can be exploited by industrialists to optimize the operation of certain machines using this kind of flow. It can be exploited in the teaching of certain units of Masters courses: gathering experimental techniques; numerical methods; and theoretical knowledge.

This work can also have a social interest where this kind of simulation can be generalized with other types of flows responsible for certain phenomena of society, such as the phenomenon of pollution. This work can have a direct impact on everyday life by the exploitation of the rotary flows, such as being a very clean and very economic means to separate the undesirable components present in certain fluid effluents.

The best contribution of this work is the new experimental and numerical database giving the effect of the aspect ratio of the cavity on the intensity of turbulence for Von Karman flow between two counter rotating disks.

A theoretical study is performed on three‐dimensional, heat transfer and fluid flow in radially rotating heated channels with steady, laminar throughflow. Consideration is given to the channel of different geometry. Both the rotational speed and throughflow rate are varied. The flow is hydrodynamically and thermally developing, with a constant wall heat flux. The velocity‐vorticity method is employed in the formulation and numerical results are obtained by means of a finite‐difference technique. The Nusselt number, friction factor, and temperature and velocity distributions are determined, and the role of the Coriolis force on the entrance‐region transport phenomena is investigated. Results are compared with the existing literature.

To investigate the effect of aspect ratio on the quantitative analogy between developing laminar flows in orthogonally rotating straight ducts and stationary curved ducts

A fractional step method is used to obtain the numerical solution of the governing equations by decoupling the solution of the momentum equations from the solution of the continuity equation. In order to clarify the similarity of the two flows, dimensionless parameters K_LR and Rossby number, Ro, in a rotating straight duct were used as a set corresponding to Dean number, K_LC, and curvature ratio, λ, in a stationary curved duct.

Under the condition that the aspect ratio was larger than one and that the magnitude of Ro or λ was large enough to satisfy the “asymptotic invariance property” the quantitative analogy between the two flows was established clearly.

As the aspect ratio decreased below one, the difference between the secondary flow intensities of these two flows increased, and therefore, the analogy between the two flows was not as evident as that for the larger aspect ratios.

Based on this methodology, the characteristics of the developing flow in orthogonally rotating ducts of higher aspect ratio can be predicted by considering the flow in stationary curved ducts, and vice versa.

The results obtained in this study will suggest an optimal criterion for the application of this approach to the flow similarity analysis in rectangular ducts with arbitrary aspect ratios.

A heated rotating cavity with an axial throughflow of cooling air is used as a model for the flow in the cylindrical cavities between adjacent discs of a high‐pressure gas‐turbine compressor. In an engine the flow is expected to be turbulent, the limitations of this laminar study are fully realised but it is considered an essential step to understand the fundamental nature of the flow. The three‐dimensional, time‐dependent governing equations are solved using a code based on the finite volume technique and a multigrid algorithm. The computed flow structure shows that flow enters the cavity in one or more radial arms and then forms regions of cyclonic and anticyclonic circulation. This basic flow structure is consistent with existing experimental evidence obtained from flow visualization. The flow structure also undergoes cyclic changes with time. For example, a single radial arm, and pair of recirculation regions can commute to two radial arms and two pairs of recirculation regions and then revert back to one. The flow structure inside the cavity is found to be heavily influenced by the radial distribution of surface temperature imposed on the discs. As the radial location of the maximum disc temperature moves radially outward, this appears to increase the number of radial arms and pairs of recirculation regions (from one to three for the distributions considered here). If the peripheral shroud is also heated there appear to be many radial arms which exchange fluid with a strong cyclonic flow adjacent to the shroud. One surface temperature distribution is studied in detail and profiles of the relative tangential and radial velocities are presented. The disc heat transfer is also found to be influenced by the disc surface temperature distribution. It is also found that the computed Nusselt numbers are in reasonable accord over most of the disc surface with a correlation found from previous experimental measurements.

This article examines the influence of centrifugal buoyancy on the hydrodynamic and thermal behaviour in fully developed flow through an orthogonally rotating duct of aspect ratio 2:1. A series of computations have been performed at rotation numbers ranging from 0 to 0.2, for constant‐density flows (no buoyancy) and also for different levels of outward and inward buoyancy. The resulting comparisons reveal that for a Reynolds number of 32,500, rotational buoyancy effects become significant at Rayleigh number values greater than 107. In outward flows, buoyancy is found to strengthen the effects of the Coriolis force on the mean motion and, by raising turbulence levels, buoyancy also enhances wall heat transfer along both the pressure and the suction side of the rotating duct. In inward flows, it is found that strong buoyancy can reverse the direction of the Coriolis‐induced secondary motion, which causes a strong rise in wall heat transfer along the suction side and a similarly significant fall in heat transfer along the pressure side. The computed effects on heat transfer are in qualitative agreement with the findings of a number of experimental studies. For both inward and outward flows, at a constant Reynolds number, the modifications of centrifugal buoyancy on the side‐averaged levels of heat transfer correlate reasonably well with the rotational Rayleigh number.

Mixed recirculatory flow in the annuli of stationary and rotating horizontal cylinders were studied numerically. A set of distorted ‘false transient’ parameters were introduced to speed up the steady state solution of the unsteady vorticity, energy and stream function—vorticity equations. The inner cylinder of the annuli is assumed heated and rotating at Reynolds numbers that exclude the effects of centrifugal acceleration and three‐dimensional Taylor vortices. The Prandtl number considered is in the range of 0.01 to 1.0 and Rayleigh number in the range of 10² to 10⁶. Radius ratios of the cylinders considered are 1.25, 2.5 and 5.0. For a radius ratio of 2.5, inner cylinder rotation in the Reynolds number range of 0 to 1120 was considered. Vertical eccentricities in the range of ±2/3 were studied for cases of the rotating inner cylinder. Numerical experiments show that the mean Nusselt number increases with Rayleigh number for both cases of concentric and eccentric stationary inner cylinder. At a Prandtl number of order 1.0 with a fixed Rayleigh number, when the inner cylinder is made to rotate, the mean Nusselt number decreases throughout the flow. At lower Prandtl number of the order 0.1 to 0.01, the mean Nusselt number remained fairly constant with respect to the rotational Reynolds number.

This study is concerned with developing laminar flow of an incompressible, Newtonian fluid, having constant viscosity, rotating in circular and rectangular ducts that contain a 180° bend. The Reynolds number ranges from 100 to 400, the rotation number from 0 to 0.4, and the Dean number from 66 to 264. Positive and negative rotation modes are considered. The artificial compressibility method is used for the numerical calculations and new boundary conditions are developed for these flows. It is shown that rotation causes the secondary flow to occur in ducts of any geometry, and that the strength of the secondary flow in the bend due to both rotation and curvature decreases as compared to the no rotation case.

The purpose of this paper is to gain a better understanding of the flow field structure around the race car in two cases: stationary wheel and rotating wheel. In addition, this paper also illustrates and clarifies the influence of wheel rotation on the aerodynamic characteristics around the race car.

The author uses steady Reynolds-Averaged Navier-Stokes (RANS) equations with the Realizable k-ε model to study model open-wheel race car. Two cases are considered, a rotating wheel and stationary wheel.

The results obtained from the study are presented graphically, pressure, velocity distribution, the flow field structure, lift coefficient (C_l) and drag coefficient (C_d) for two cases and the significant influence of rotating case on flow field structure around wheel and aerodynamic characteristics of race car. The decreases in C_d and C_l values in the rotating case for the race car are 16.83 and 13.25 per cent, respectively, when compared to the stationary case.

Understanding the flow field structures and aerodynamic characteristics around the race car in two cases by the steady RANS equations with the Realizable k-ε turbulence model.

A numerical study of a quantitative analogy of fully developed turbulent flow in a straight square duct rotating about an axis perpendicular to that of the duct and a stationary curved duct of square cross‐section was carried out. In order to compare the two flows, the dimensionless parameters K_TR=Re^1/4/√Ro and the Rossby number, Ro=w_m/Ωd_h, in the rotating straight duct flow corresponded to K_TC=Re^1/4/√λ and the curvature ratio, λ=R/d_h, in the stationary curved duct flow, so that they had the same dynamical meaning as those parameters for fully developed laminar flow. For the large values of Ro or λ, the flow field satisfied the “asymptotic invariance property”; there were strong quantitative similarities between the two flows, such as in the flow patterns and friction factors for the same values of K_TR and K_TC. Based on these similarities, it is possible to predict the flow characteristics in rotating ducts by considering the flow in stationary curved ducts, and vice versa.